In 1991, Sony announced new class of lithium secondary batteries, called lithium-ion batteries, which strongly impacted the battery community all over the world because of their high operating voltage. It consists of two lithium intercalation materials, one for the negative electrode and the other for the positive electrode in an electrochemical cell. Combination of the two lithium insertion materials performs the basic function of lithium-ion batteries. The development of lithium-ion rechargeable batteries of high energy density relies on the successful development of high energy density intercalation electrode materials.
References may be made to patent “US 2006/0078797” wherein the use of graphite anode and Li8V6O13 cathode which gives the energy density of 140-280 Wh/kg.
U.S. Pat. No. 7,736,809 B2 discloses the layer-by-layer assembly of the cathode active a material on the current collector and the anode is the lithium deposited copper which provides the energy density of 140-180 Wh/kg.
References may be made to patent “U.S. Pat. No. 7,104,637 B2” wherein the lithium ion battery delivers the capacity retention of 70-75% after 500 cycles which uses the conventional active materials for anode and cathode, the separator is the porous elastomer.
Carbon based anode materials, primarily petroleum coke has been considered as intercalating anode material with 372 mAh/g as specific capacity. In the present day lithium ion batteries carbon is normally coated on to a copper foil current collector. This material delivers only 150-170 mAh/g in a practical cell.
References may be made to patent “U.S. Pat. No. 7,285,358 B2” wherein the carbon source comprises an amorphous carbon or a crystalline carbon which has a thickness ranging from 1 to about 300 nm and produces good electrochemical performance.
References may be made to patent “US 2009/0117467 A1” wherein a nano-scaled graphene platelet-based composite material, which exhibits an exceptional specific capacity, an excellent reversible capacity and a long cycle-life has been disclosed.
References may be made to patent “U.S. Pat. No. 6,503,660 B2” wherein crystalline graphitic carbon nano fibers composed of graphite sheets provides highest discharge capacity and cycling stability has been disclosed. However, all the above graphite based anode materials exhibit large amount of irreversible capacity.
Lithiated transition metal oxides, namely LiCoO2, LiMn2O4 and LiNiO2 are primarily considered as cathode materials for lithium ion batteries. LiNiO2 possess safety problem during charging, causes difficulties such as excessive evolution of gas, build-up of cell resistance, decomposition of electrolyte etc. LiMn2O4 is one of the eco-friendly cathode materials that possess many advantages such as low cost, safety, price and low toxicity. The main disadvantage is structural instability. Because of the above problems researchers mainly focused to improve the performance of the LiCoO2 cathode materials. LiCoO2 cathode material, first introduced by Mizushima et al “LixCoO2 (0<x<1): A new cathode material for batteries of high energy density-K. Mizushima, P. C. Jones, P. J. Wiseman, J. B. Goodenough, Mater. Res. Bull. 15 (1980) 783” in 1980's, with a layer structure is used as the preferred cathode material in majority of the commercial lithium ion cells in view of its ease of synthesis and high reversibility though active studies are also being made towards its improvement.
As charge and discharge takes place, the valence number of Co in LiCoO2 changes as follows:Li+Co3+O22−→Li++Co4+O22−+e−
As apparent from this formula, LiCoO2 contains trivalent Co and brings about discharging as the trivalent Co is changed to tetravalent Co. Substituting another element, which has small size, compared to Co3+ ion leads to contraction along the c-axis results in mechanical failure of the LiCoO2 particles and rapid capacity fading “Al2O3 coated LiCoO2 as cathode material for lithium ion batteries—L. Liu, Z. Wang, H. Li, L. Chen, X. Huang, Solid-State Ionics 152-153 (2002) 341-346”. Therefore difficult to acquire a highly reliable positive active material with high discharge capacity and low capacity fade.
Cho et al “Novel LiCoO2 cathode material with Al2O3 coating for lithium ion cell-J. Cho, Y. J. Kim, B. Park, Chem. Mater. 12 (2000) 3788” and “Zero strain intercalation cathode for rechargeable Li-Ion cell—J. Cho, Y. J. Kim, B. Park, Angew. Chem., Int. Ed. 40 (2001) 3367” and many other researchers achieved the reversible capacity at high voltages through coating with inactive metal oxides thereby providing good structural stability during cycling. Another approach is to improve the structural stability as well as cycling stability of the LiCoO2 materials through doping with transition metals cations and non-transition metal cations “Synthesis and electrochemical performance of tetravalent doped LiCoO2 in lithium rechargeable batteries—S. Gopukumar, Yonghyun Jeong, Kwang Bum Kim, Solid-State Ionics 159 (2003) 223-232”, “Performance of LiM0.05Co0.95O2 cathode materials in lithium rechargeable cells when cycled up to 4.5V—Meijing Zou, Masaki Yoshio, S. Gopukumar, and Jun-ichi Yamaki, Chem. Mater. 17 (2005) 1284”, “Microwave assisted synthesis and electrochemical behaviour of LiMg0.1Co0.9O2 for lithium rechargeable batteries—C. N. Zaheena, C. Nithya, R. Thirunakaran, A. Sivashanmugam, S. Gopukumar, Electrochim. Acta 54 (10) (2009) 2877-2882”, “On the LixCo1−yMgyO2 system upon deintercalation; electrochemical, electronic properties and 7Li MAS NMR studies—S. Levasseur, M. Menetrier, C. Delmas, J. Power Sources 112 (2) (2002) 419-427”, “Preparation and electrochemical characterization of lithium cobalt oxide nano particles by modified sol-gel method—Ramdas B. Khomane, Amit C. Agrawal, B. D. Kulkarni, S. Gopukumar, A. Sivashanmugam, Mater. Res. Bull. 43 (2008) 2494” and “Influence of Aluminium doping on the properties of LiCoO2 and LiNi0.5Co0.5O2 oxides—S. Castro-Garcia, A. Castro-Coucerio, M. A. Senaris-Rodriguez, F. Soulette, C. Julien, Solid-State Ionics 156 (2003) 15-26”. Many patents and publications have suggested this coating approaches but this improvement to commercial batteries did not sufficiently solve the high voltage and elevated temperature problems. H. Y. Xu et al “Microwave assisted synthesis and electrochemical behaviour of LiMg0.1Co0.9O2 for lithium rechargeable batteries—C. N. Zaheena, C. Nithya, R. Thirunakaran, A. Sivashanmugam, S. Gopukumar, Electrochim. Acta 54 (10) (2009) 2877-2882”, W. Luo et al “Synthesis, characterization and thermal stability of LiNi1/3Mn1/3Co1/3-zMgzO2, LiNi1/3-zMn1/3Co1/3MgzO2, LiNi1/3Mn1/3-zCo1/3MgzO2—W. Luo, F. Zhou, X. Zhao, Z. Lu, X. Li, J. R. Dahn, Chem. Mater. 22 (2010) 1164” and R. Vasanthi et al “Synthesis and characterization of non-stoichiometric compound LiCo1−x−yMgxAlyO2 (0.03≦x and y≦0.07) for lithium battery application—R. Vasanthi, I. Ruthmangani, S. Selladurai, Inorg. Chem. Commun. 6 (2003) 953” reported that Mg is one of the effective dopant, which increases structural stability during cycling due to the pillaring effect. However, upper cut-off voltage increases >4.3 V Vs Li/Li+, capacity gradually decreases due to the non-suitable co-dopant. Deepa et al “Synthesis and electrochemical behaviour of copper doped manganate and cobaltate cathode material for lithium batteries—S. Deepa, N. S. Arvindan, C. Sugadev, R. Tamilselvi, M. Sakthivel, A. Sivashanmugam, S. Gopukumar, Bull. Electrochem. 15 (1999) 381-384” and M. Zou et al “Synthesis of High-voltage (4.5V) cycling doped LiCoO2 for use in lithium rechargeable cells—Meijing Zou, Masaki Yoshio, S. Gopukumar, Jun-ichi Yamaki, Chem. Mater. 15 (15) (2003) 4699-4702” investigated Cu as one of the effective dopant to increase the cycling stability at high voltages.
The present day lithium ion batteries are normally charged up to 4.2V and provide the discharge capacity of 130-140 mAhg−1. Increasing the charging voltage of lithium cobalt oxide based batteries to 4.3, 4.4 and 4.5 V Vs Li/Li+ will significantly increase the reversible capacity to 160, 170 and 190 mAhg−1. Cho et al “Novel LiCoO2 cathode material with Al2O3 coating for lithium ion cell-J. Cho, Y. J. Kim, B. Park, Chem. Mater. 12 (2000) 3788” achieved this reversible capacity at high voltages through coating with inactive metal oxides thereby providing good structural stability during cycling.
However, coating approaches did not sufficiently solve the high voltage and elevated temperature problems to commercial batteries. In order to overcome these problems, elemental substitution is one of the best approaches to achieve the highest reversible capacity. Many researchers “The negative and positive structural effects of Ga doping in the electrochemical performance of LiCoO2—S. M. Lala, L. A. Montoro, V. Lemos, M. Abbate, J. M. Rosolen, Electrochim. Acta 51 (2005) 7-13”, “Synthesis and electrochemical performance of tetravalent doped LiCoO2 in lithium rechargeable batteries-S. Gopukumar, Yonghyun Jeong, Kwang Bum Kim, Solid-State Ionics 159 (2003) 223-232”, “Structural and thermal properties of LiNi0.6−xMgxCo0.25Mn0.15O2 cathode materials—P. Y. Liao, J. G. Duh, H. S. Sheu, J. Power Sources 183 (2008) 766”, “Microwave synthesis and electrochemical properties of LiCo1−xMxO2 (M=Al and Mg) cathodes for Li-ion rechargeable batteries—P. Elumalai, H. N. Vasan, N. Munichandraiah, J. Power Sources 125 (2004) 77, Effects of Sn doping on the structural and electrochemical properties of LiNi0.8Co0.2O2 cathode materials-Xiaoling Ma, Chiwei Wang, Jinguo Cheng, Jutang Sun, Solid State Ionics 178 (2007) 125” and “La doped LiCoO2 with high rate capability—P. Ghosh, S. Mahanty, R. N. Basu, Electrochim. Acta 54 (2009) 1654” investigated doping metal cations such as Ti, Zr, Mg, Ni, Al, Sn, Ga and rare earth metal ions such as La etc. References may be made to patent US 2004-91780 A1, wherein addition of LiF and LiOH to mixed hydroxides prior to a solid state reaction is disclosed. The present invention discloses a new material with high capacity and better cyclability by adopting mixed dopent approach involving specific cation combination.
References may be made to patent “US 2002-14222 A1” wherein doping of halogens to high crystalline LiCoO2. However, these halogens do not act as good dopant because the addition of LiF initially increases capacity but gradually decreases has been disclosed. Addition of MgF2 might be suitable for spinel or Li—Ni—Mn—Co based materials, but it is not recommended for LiCoO2. U.S. Pat. No. 6,613,479 discloses the fluorine doping to LiMnO2 material, however, the material are to be prepared in inert gas at low temperature and also it has poor crystallinity.
Due to the many problems in the above approaches, mixed dopant approach is found to be the best method, which provides better cycling performance as suggested by few researchers “Synthesis, characterization and thermal stability of LiNi1/3Mn1/3Co1/3-zMgzO2, LiNi1/3-zMn1/3Co1/3MgzO2, LiNi1/3Mn1/3-zCo1/3MgzO2—W. Luo, F. Zhou, X. Zhao, Z. Lu, X. Li, J. R. Dahn, Chem. Mater. 22 (2010) 1164”, “Synthesis and characterization of non-stoichiometric compound LiCo1−x−yMgxAlyO2 (0.03≦x and y≦0.07) for lithium battery application—R. Vasanthi, I. Ruthmangani, S. Selladurai, Inorg. Chem. Commun. 6 (2003) 953” and “Effects of Sn doping on the structural and electrochemical properties of LiNi0.8Co0.2O2 cathode materials—Xiaoling Ma, Chiwei Wang, Jinguo Cheng, Jutang Sun, Solid State Ionics 178 (2007) 125”.
Mg has been reported to be one of the effective dopant, which increases structural stability during cycling due to the pillaring effect. However, cut-off voltage above 4.3 V Vs Li/Li+, decreases the capacity gradually due to the non-compatible co-dopants. Deepa et al “Synthesis and electrochemical behaviour of copper doped manganate and cobaltate cathode material for lithium batteries—S. Deepa, N. S. Arvindan, C. Sugadev, R. Tamilselvi, M. Sakthivel, A. Sivashanmugam, S. Gopukumar, Bull. Electrochem. 15 (1999) 381-384” and M. Zou et al “Performance of LiM0.05Co0.95O2 cathode materials in lithium rechargeable cells when cycled up to 4.5V—Meijing Zou, Masaki Yoshio, S. Gopukumar, and Jun-ichi Yamaki, Chem. Mater. 17 (2005) 1284” investigated Cu as one of the effective dopant to increase the cycling stability at high voltages.